The present invention relates to an image reader.
An image reader which radiates light onto an original placed on a document table and reads, as image data, the light reflected from or passed through the original by means of image pickup device, such as a CCD, has been generally known.
In the case of such an image reader, electric charges corresponding to the quantity of the light reflected from the original are stored in the image pickup device. The thus-stored electric charges are output from the image pickup device as an electric signal corresponding to the quantity of electric charge at predetermined timing. Since the electric signal output from the image pickup device is an analog electric signal, the signal is converted into a digital electric signal by an analog-to-digital converter. The thus-converted electric signal is subjected to various modifications performed by an image data preparation circuit (corresponding to an image data preparation unit in claims), and the resultant signal is output as image data to, e.g., an external personal computer.
The above-described image reader utilizes a clock signal for setting an output timing at which an electric signal is output from image pickup device and a data processing timing at which image data are prepared from the thus-output electric signal. In synchronism with the clock timing of the clock signal, the image pickup device outputs, as an electric signal, the electric charges stored in each pixel of the image pickup device. In accordance with the clock timing of the clock signal, the image data preparation circuit performs processing, such as computation of image data.
With a view toward energy saving, demand exists not only for an image reader involving smaller power consumption but also for a computer peripheral device involving smaller power consumption. In the case of an image reader, after lapse of a predetermined period of time after an image reading operation has been completed, a lamp acting as the light source is extinguished or supply of power to a peripheral circuit, such as an image data preparation circuit, is stopped, thus curtailing power consumption except of that required for performing an image reading operation.
However, demand for energy saving has recently become increased more than ever, and power consumption must be curtailed further.
In order to save energy, there is a conceivable measure for halting supply of power to image pickup device. However, if supply of power to the image pickup device is halted, the image pickup device must be warmed up before reading an original. For this reason, a predetermined period of time is required from the time a read start instruction is issued until a reading operation is started, which in turn deteriorates the ease of operation of the image reader.
A clock signal is supplied to the image pickup device at all times. To this end, power must be supplied to the clock generation circuit for producing a clock signal. When a clock signal is supplied to image pickup device, the image pickup device is activated by the thus-supplied clock signal, to thereby generate heat. A charge coupled device, such as a CCD, employed in the image pickup device generates minute heat and also produces a so-called dark current in accordance with the quantity of heat. When an original is read, the thus-produced dark current is included in image data as noise, which may worsen the quality of the thus-read image.
As a result of supply of a clock signal, the image pickup apparatus generates Electromagnetic Interference (EMI), and the EMI worsens the quality of the thus-read image.